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BELL SYSTEM TECHNICAL JOURNAL 



Miller's method is illustrated in Fig. 13. To determine n the key 

 Ki is open and Ki is closed. The resistance r\ is adjusted until the 

 sound heard in the telephones T is a minimum, under which circum- 

 stances it is clear that u=— . To determine r p , some definite relation 



between r\ and ri is established. Then, with key K\ closed, the re- 

 sistance r is adjusted until the telephone response is a minimum. 

 With this adjustment it may be shown that 



r P = r U^-l\. (3) 



This measurement of r p may be simplified as follows: suppose we 



adjust Y\ for a minimum tone in T when K\ is open. Then n=— > 



and it is seen from Equation 3 that with this relation between r\ and 

 r 2 it would not be possible to obtain a balance with K\ closed; but if 



Fig. 13 



r 2 be doubled, which can be done by opening Ki thus adding a re- 

 sistance equal to ri, and r be now adjusted with K\ closed to give a 

 minimum tone in T, then r P — r . 



12. Dynamic Characteristics of Vacuum Tubes. In a circuit such as 

 that shown in Fig. 13, the space current causes a fall of potential 

 along any resistance r, and the difference in potential between filament 

 and plate is therefore less than the potential difference across the 

 battery by the amount I P r. If I p is increased in any way, as for 

 instance, by an increase in E g , the drop I p r increases and with a fixed 

 battery e.m.f. the potential difference between the filament and plate 

 diminishes somewhat. It follows, therefore, that a given change in 

 Eg will cause a smaller change in space current when the plate circuit 

 includes an external resistance r than when it does not. 



This important fact supplies a simple means of straightening the 

 characteristic of a vacuum tube to such an extent that it may become 

 practically a distortionless amplifier. 



